Generally, zeolite is widely used as a catalyst, an adsorbent, a molecular sieve, an ion exchanger or the like because it has a peculiar three-dimensional crystal structure of aluminosilicate, and has large pore and excellent ion-exchange performance compared to other aluminosilicate crystals. The use of natural zeolite is limited because of its structural restrictions, but the use of synthetic zeolite is gradually enlarged. In order to expand the use of zeolite, it is required to arbitrarily control the crystal size, particle size distribution and form of zeolite as well as to efficiently synthesize zeolite.
ZSM-5 zeolite forms three-dimensional pores defined by 10 tetrahedron rings, and its size is equal to that of zeolite A or is in the middle between zeolite X and zeolite Y. Further, ZSM-5 zeolite is a kind of pentasil zeolite which is a shape-selective catalyst exhibiting peculiar adsorption and diffusion characteristics, and generally has high thermal stability and has hydrophobicity because it has high ratio of SiO2/Al2O3. Further, ZSM-5 zeolite has strong Lewis acid sites, but has weak Bronsted acid sites. In particular, ZSM-5 zeolite is used to directly obtain gasoline fraction having a high octane number from methanol by an MTG process, and is known to have excellent selectivity of gasoline fraction.
After ZSM-5 having a high content of silica was first developed by Mobil Corporation in the early 1970's, research into this material has been variously made due to peculiar catalytic activity and shape selectivity resulting from the molecular sieve effect of this material. Unlike aluminosilicate zeolite, various kinds of organic materials have been used as structure inducing substances for forming a structure to prepare ZSM-5.
To date, among the organic materials known to be effective in forming the structure of ZSM-5, tetrapropylammonium cations have been known to have the most excellent structure inducing effect. Currently, most of commercially-available ZSM-5 is being synthesized using this material. However, although organic structure inducing materials including tetrapropylammonium ions exhibit excellent structure inducing effects, attempts not to use them have been made because they are disadvantageous in the economical and environmental aspects, and several processes for this purpose have been developed (U.S. Pat. No. 4,257,885). The reason for not using organic structure inducing materials is that they are expensive and that they cause environmental pollution because they have strong toxicity. When ZSM-5 is synthesized using an organic structure inducing material, a secondary cost is required to treat a toxic unreacted organic structure inducing material included in waste water.
Further, the structure inducing material included in the crystalline particles of the synthesized ZSM-5 must be pyrolyzed and then removed by a calcination process at 550° C. or more. In this case, when the structure inducing material is not completely pyrolyzed during the calcination process, pores are blocked, thus severely deteriorating catalytic activity. Further, an additional cost is required due to the calcination process, and atmospheric pollution cannot be avoided due to exhaust gas generated by the pyrolysis of organic materials.
Therefore, in order to overcome the above problems, Flanigen et al. (U.S. Pat. No. 4,257,885) first reported a method of synthesizing ZSM-5 using crystalline seeds under the condition of excluding an organic structure inducing material or without using crystalline seeds. However, this synthesis method is problematic in that it needs a long reaction time of 68˜120 hours. Further, when ZSM-5 is synthesized under the condition of excluding the organic structure inducing material, this method is sensitively influenced by reaction conditions, thus requiring careful attentions.
The factors influencing the synthesis of ZSM-5 may include the type of a silica source, the ratio of Si/Al, the concentration of and alkali solution, the mixing sequence of reactants, crystallization temperature, crystallization time, the degree of aged, stirred or not, and the like. Among these factors, the type of a silica source is known as the most important factor.
Water glass, silica sol or the like is used as the silica source. Water glass, which is prepared by melting solid silicate with water, is the cheapest silica source. However, it is difficult to control the composition of reactants because water glass includes a large amount of alkali components. Therefore, the concentration of alkali components in the water glass can be controlled by the addition of sulfuric acid or aluminum sulfate. However, this synthesis method is problematic in that ZSM-5 is nonuniformly crystallized because reaction conditions are complicated, and in that the cost for post-treatment such as salt removal is increased (related German Patent No. 207185).
Silica sol, which is another silica source, has good reactivity and is easily treated. However, silica sol is more expensive than other silica sources, and its silica components are dispersed in a large amount of water in a colloidal state and react with aluminum components to form hydrogel, so that the two components must be brought into contact with each other in a diluted state in order to prevent the formation of hydrogel. In this case, there are problems in that the solid content of the synthesized ZSM-5 is low based on the particles crystallized during the process of synthesizing ZSM-5, and the crystalline particles of ZSM-5 are finely dispersed in a state of separate particles, so that a high load occurs during a remainder separation process and a water washing process, and in that unreacted components are discharged from the remainder and the water washing solution, with the result that the productivity of ZSM-5 becomes low, so that this synthesis method is not suitable for industrial production methods (related German Patent No. 207186).
In addition, Korean Unexamined Patent Application Publication No. 10-2007-0020354 discloses a method of preparing a ZSM-5 molecular sieve catalyst having a small crystal size using diatomite or silica aerogel as a main silica source by adding a seed crystal orienting agent, silica sol and sodium silicate to conduct kneading and molding and then performing a gas-solid phase crystallization of the silica source using organic amine and steam to convert the crystallized silica source to ZSM-5 having a small crystal size. However, this method is also problematic in that process costs are increased because nanosized seeds and organic amine are used in order to obtain fine ZSM-5.
Further, Korean Patent registration No. 1996-0002621, filed by Mobil Corporation, discloses a method of preparing small-crystal-sized ZSM-5 having high mesitylene absorbing ability without adding any organic material. In this method, ZSM-5 is prepared by using a reaction mixture including an alumina source, acid and ZSM-5 seeds in addition to sodium silicate used as a silica source under the condition that an organic structure inducing material does not exist. This method is characterized in that the crystal size of ZSM-5 is controlled using the solid content of the reaction mixture and the molar ratio of OH—/SiO2, but is problematic in that the degree of crystallization of ZSM-5 does not reach 50˜75%.
Meanwhile, recently, as a method for shortening hydrothermal synthesis time, a microwave synthesis method is introduced. In the microwave synthesis method, the time taken to form seeds and crystallize a sample can be shortened by directly supplying microwave energy to the sample not by supplying energy from an external heat source to the sample using thermal conduction. That is, ions are rapidly vibrated and water dipoles are rapidly rotated by microwaves, so that temperature is rapidly raised by the friction between molecules in a solution, thereby rapidly crystallizing the sample.
Mobil Corporation of U.S.A. first introduced a method of preparing a porous molecular sieve material using microwave energy (U.S. Pat. No. 4,778,666). In this method, the microwave energy used to synthesize the zeolite had a frequency range of 915˜2450 MHz, and ZSM-5 zeolite was synthesized using crystal seeds in a container (glass, ceramic, PTF). Recently, methods of synthesizing nanosized silicalite-1, ZSM-5, LTL, LTA and the like by dividing reactions into two steps of a seed formation reaction and a crystallization reaction and then applying microwaves thereto have been reported by Nan Ren and Yi Tang et al. (Microporous and Mesoporous Materials, 3, 306 (2009)).
As described above, the above-mentioned ZSM-5 synthesis methods are problematic in that, when ZSM-5 is synthesized using cheap water glass as a silica source without an organic structure inducing material, the composition range of synthesizable reactants is narrow, and synthesis time is long. Further, the above-mentioned ZSM-5 synthesis methods are problematic in that the distribution of particle sizes is wide, and the degree of crystallinity of synthesized zeolite is low.